A tunable Yttrium-Iron-Garnet (YIG) oscillator is an oscillator that generates signals in the microwave band from 500 MHz up to 40 GHz. The heart of the YIG oscillator is a YIG spherical resonator. A YIG spherical resonator has a natural resonant frequency that is proportional to the strength of the magnetic field going through the YIG spherical resonator. YIG resonators resonate when a magnetic field is applied to a YIG (Y.sub.3 Fe.sub.5 O.sub.12) sphere. The output frequency of a YIG oscillator is a function of: (1) the external magnetic field strength that is supplied to the YIG sphere by an associated permanent magnet and (2) a small anisotropy field in the YIG sphere that can add to or subtract from the external magnetic field strength depending on the orientation of the crystal axes of the YIG sphere to the external magnetic field.
YIG oscillators appeal to customers because they generate an output signal that is very clean. A YIG oscillator that is specified to have an output frequency of 5 GHz will output a frequency of 5 GHz with very little deviation. YIG oscillators are stable and have little jitter. They also have little phase noise, which is a measure of spectral purity. An oscillator which is not spectrally clean or which has a lot of jitter is going to have trouble if another signal is placed close to the oscillator signal. If the oscillator signal has a lot of noise, it will smear into another signal that is of importance. Noisy oscillators force designers to space other oscillators further away. In the telecommunications world, bandwidth is critical, and there are only certain bands allocated for microwave radios. If an oscillator has a lot of jitter and a lot of phase noise, then the oscillator will interfere with another oscillator having a signal operating in a nearby band.
Oscillators have important applications in cellular telephones. As the cellular telephone community expands, the applications for YIG oscillators will continue to increase. Cellular telephones require base stations to receive, amplify, and transmit communications signals. The base station receiver is basically a radio receiver that requires a local oscillator. This local oscillator has to be a very clean, low-phase noise, local oscillator.
Another oscillator called a dielectric resonator oscillator competes with the YIG oscillator. The dielectric resonator oscillator is also a very low phase noise oscillator. The difference between the dielectric resonator oscillator and the YIG oscillator is that the YIG oscillator is tunable, which enables the frequency of the YIG oscillator to be changed.
For example, if a company is building a base station transceiver, it is allocated a certain frequency or band of frequencies. If the company buys an oscillator that has to be set at 5 GHz, the company can either buy a YIG oscillator or a dielectric resonator oscillator. If the FCC reallocates frequencies, the company can no longer use the local oscillator at 5 GHz. With the dielectric resonator oscillator, a technician would have to physically go into the radio; i.e. physically remove the dielectric resonator oscillator, replace it with a totally different dielectric resonator oscillator, and then test it to assure everything works. A skilled and expensive technician must do the work. Moreover, the company would have to stock all kinds of dielectric resonator oscillators that have different oscillating frequencies because the frequencies may change again.
YIG oscillators on the other hand, unlike the dielectric resonator oscillators, are tunable or frequency agile. Supplying a little current to the oscillator enables the frequency of the oscillator to shift from 4 GHz to 6 GHz or to any frequency in between. Referring to the base station transceiver example, a simple software manipulation can complete the frequency transformation for the company. The software manipulation does not even necessarily have to take place at the base station. It can be accomplished remotely.
A single YIG oscillator could replace multiple dielectric resonator oscillators that a customer might have to stock because the customer does not know what frequency may be required in the future.
Temperature stability in a YIG oscillator is very important. Excessive temperature drift can defeat the YIG oscillator's primary purpose that is to serve as a stable frequency source of microwave energy. There are several sources of temperature drift that can lead to a change in output frequency with changing oscillator case temperature. The permanent magnet field will change as a function of temperature, as will several magnetic properties of the high permeability steel used for the magnetic circuit return path and shield. The magnet properties of a particular permanent magnet are fixed. So are the properties of the YIG housing or flux guide. They will have certain field vs. temperature characteristics that are fixed.
The small anisotropy field in a YIG sphere is very temperature dependent and substantially influences the output frequency of the YIG spherical resonator. It has been found that the YIG spherical resonator has frequency drift versus temperature characteristics that are a function of where a zero temperature compensation (ZTC) axis of the YIG sphere lies in relation to the direction of the magnetic field supplied by the permanent magnet. The YIG sphere can have positive, negative or zero temperature coefficients.
Manufacturing YIG oscillators involves a number of steps. First, both of the 111 crystallographic axes of the YIG sphere are aligned with the plane of the external magnet field, which is generated by the electromagnet using a YIG sphere orienter. Next, the aligned YIG sphere is permanently attached by epoxy to one end of a cylindrical sphere rod. The YIG sphere and the sphere rod are then inserted into the housing of the YIG oscillator through a hole in a rod holder that is formed of a metal block. The sphere rod includes a small slot in the end opposite the YIG sphere so it can be rotated through 360 degrees with a small screwdriver blade external to the housing. The length of the rod is such that after being inserted into the housing, the YIG sphere sits directly under an oscillator circuit-coupling loop. The coupling loop is itself situated in-between the pole pieces of a permanent magnet. Next, the YIG sphere is rotated until its ZTC axis is aligned with the direction of the magnetic field of the permanent magnet. Finally, the end of the sphere rod closest to the sphere is then secured to the housing using epoxy.
There are a number of problems associated with the manufacturing of the prior art YIG oscillators.
First, a rod holder is required so that the sphere rod can be rotated. The rod holder is expensive to make and adds cost to the oscillator.
Second, the YIG sphere and the rod holder combination is susceptible to vibration, leading to vibration-induced frequency modulation (FM) noise. Since the YIG sphere sits at the end of a long sphere rod that is cantilevered in space, it will vibrate with the sphere rod when the oscillator is under external shock. YIG sphere vibration inside the coupling loop translates directly into unwanted noise at the oscillator output.
Third, the rod holder must be precisely positioned so that the YIG sphere at the end of the sphere rod is centered within the coupling loop. The manual precision positioning of the rod holder is time consuming and adds to the cost of the oscillator.
Fourth, the rod holder requires additional space in the oscillator. This is undesirable, particularly for more advanced, miniature YIG oscillators where space is at a premium.
Fifth, to prevent the sphere rod from vibrating, the end of the sphere rod closest to the YIG sphere is secured to the housing using epoxy. This is a time consuming step, both in the application of the epoxy and the time needed for it to cure. No other work can be done to the partially assembled oscillator until the epoxy has cured. This increases the manufacturing cycle time and undesirably increases the manufacturing cost due to the additional work-in-process inventory.
Finally, an access hole has to be machined into the side of the oscillator housing to allow access by a small tool, such a screwdriver blade to adjust the position of the sphere rod and the YIG sphere with respect to the magnetic field. After the YIG sphere alignment is completed, this hole must be sealed either by welding or epoxy. This is a time-consuming step that increases the manufacturing cycle time and manufacturing cost due to the additional work-in-process inventory.
As the number of YIG oscillators required by customers increases, it becomes more desirable to develop apparatus and manufacturing methods for making YIG oscillators more efficiently and less expensively.